Geology
By the end of the last ice age about 11,000 years ago, much of northern Europe and North America was covered by ice sheets up to 3 km thick. At the end of the ice age when the glaciers retreated, the removal of the weight from the depressed land led to a post-glacial rebound. Initially the rebound was rapid, proceeding at about 7.5 cm/year. This phase lasted for about 2,000 years, and took place as the ice was being unloaded. Once deglaciation was complete, uplift slowed to about 2.5 cm/year, and decreased exponentially after that. Today, typical uplift rates are of the order of 1 cm/year or less, and studies suggest that rebound will continue for about another 10,000 years. The total uplift from the end of deglaciation can be up to 400 m.
In the Viking Age Mälaren was still a bay of the Baltic Sea, and seagoing vessels could sail up it far into the interior of Sweden. Birka was conveniently near the trade routes through the Södertälje Canal. Due to the post-glacial rebound, Södertälje kanal and the mouth of Riddarfjärden bay had become so shallow by about the year 1200 that ships had to unload their cargoes near the entrances, and progressively the bay became a lake. The decline of Birka and the subsequent foundation of Stockholm at the choke point of Riddarfjärden were in part due to the post-glacial rebound changing the topography of the Mälaren basin. The lake's surface currently averages 0.7 meters above sea level.
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